Liquid crystal display device

ABSTRACT

Pixels of a liquid crystal display device exhibit, in a switched manner, a black display state where black display is provided in a state where a vertical electric field is generated in a liquid crystal layer, a white display state where white display is provided in a state where a lateral electric field is generated in the liquid crystal layer, and a transparent display state where a rear side of a liquid crystal display panel is seen through where no voltage is applied to the liquid crystal layer. A gray scale level group including gray scale levels from a lowest level to a highest level includes a white level, a transparent level having a luminance higher than that of the white level, and a plurality of sub-transparent levels each having a luminance higher than that of the white level and lower than that of the transparent level.

TECHNICAL FIELD

The present invention relates to a liquid crystal display device, andspecifically, to a liquid crystal display device preferably usable as asee-through display device.

BACKGROUND ART

Recently, a see-through display device is a target of attention as adisplay device for information display or digital signage. Such asee-through display device allows the background (rear side of a displaypanel) to be seen through, and thus is capable of displaying informationdisplayed on the display panel and the background in an overlappingmanner. Therefore, the see-through display device has a splendid effectof appealing to potential customers and a splendid eye-catching effect.It has also been proposed to use the see-through display device for ashowcase or a shop window.

In the case where a liquid crystal display device is used as asee-through display device, there is a bottleneck that the liquidcrystal display device has a low light utilization factor. Such a lowlight utilization factor is caused by a color filter or a polarizationplate provided in a general liquid crystal display device. The colorfilter and the polarization plate absorb light of a specific wavelengthrange or light of a specific polarization direction.

In such a situation, it is conceivable to use a liquid crystal displaydevice of a field sequential system. In the field sequential system, thecolor of light directed from an illumination element toward a liquidcrystal display panel is switched in a time division system to providecolor display. Therefore, the color filter is not needed, and thus thelight utilization factor is improved. However, the field sequentialsystem requires a liquid crystal display device to have a high speedresponse.

Patent Document 1 and Patent Document 2 each disclose a liquid crystaldisplay device having an improved response characteristic by includingan electrode structure capable of generating a vertical electric fieldand a lateral electric field in a switched manner in a liquid crystallayer. In the liquid crystal display device disclosed in each of PatentDocument 1 and Patent Document 2, a vertical electric field is generatedin the liquid crystal layer in one of a transition from a black displaystate to a white display state (rise) and a transition from the whitedisplay state to the black display state (fall), and a lateral electricfield (fringe field) is generated in the liquid crystal layer in theother of the rise and the fall. Therefore, a torque by voltageapplication acts on liquid crystal molecules in both of the rise and thefall, and thus a high speed response characteristic is provided.

Patent Document 3 also proposes a liquid crystal display devicerealizing a high speed response by causing an alignment control force,provided by an electric field, to act on the liquid crystal molecules inboth of the rise and the fall.

CITATION LIST Patent Literature

-   Patent Document 1: PCT Japanese National-Phase Laid-Open Patent    Publication No. 2006-523850-   Patent Document 2: Japanese Laid-Open Patent Publication No.    2002-365657-   Patent Document 3: WO2013/001979

SUMMARY OF INVENTION Technical Problem

However, it has been found that use of the liquid crystal display devicedisclosed in each of Patent Documents 1, 2 and 3 as a see-throughdisplay device causes a problem that the background is blurred (visuallyrecognized double or seen double) for the reasons described below indetail and thus the display quality is declined. Patent Document 1, 2 or3 does not described such a use (application as a see-through displaydevice). The occurrence of the above-described problem is knowledgenewly found by the present inventors.

The present invention made in light of the above-described problem hasan object of providing a liquid crystal display device that has a highresponse characteristic and also provides a high display quality and ispreferably usable as a see-through display device.

Solution to Problem

A liquid crystal display device in an embodiment according to thepresent invention includes a liquid crystal display panel including afirst substrate and a second substrate facing each other, and a liquidcrystal layer provided between the first substrate and the secondsubstrate; the liquid crystal display device including a plurality ofpixels arrayed in a matrix. The first substrate includes a firstelectrode provided in each of the plurality of pixels and a secondelectrode generating a lateral electric field in the liquid crystallayer together with the first electrode. The second substrate includes athird electrode provided to face the first electrode and the secondelectrode, the third electrode generating a vertical electric field inthe liquid crystal layer together with the first electrode and thesecond electrode. The plurality of pixels each exhibit, in a switchedmanner, a black display state where black display is provided in a statewhere the vertical electric field is generated in the liquid crystallayer, a white display state where white display is provided in a statewhere the lateral electric field is generated in the liquid crystallayer, and a transparent display state where a rear side of the liquidcrystal display panel is seen through in a state where no voltage isapplied to the liquid crystal layer. A gray scale level group includinggray scale levels from a lowest level to a highest level includes awhite level corresponding to the white display state, a transparentlevel corresponding to the transparent display state and having aluminance higher than that of the white level, and a plurality ofsub-transparent levels each having a luminance higher than that of thewhite level and lower than that of the transparent level.

In an embodiment the second electrode is provided below the firstelectrode with an insulating layer being provided between the firstelectrode and the second electrode.

In an embodiment, a potential difference between the first electrode andthe second electrode at each of the plurality of sub-transparent levelsis smaller than a potential difference between the first electrode andthe second electrode at the white level; and a potential differencebetween the second electrode and the third electrode at each of theplurality of sub-transparent levels is smaller than a potentialdifference between the second electrode and the third electrode at thewhite level.

In an embodiment, at the plurality of sub-transparent levels, as thegray scale level is increased, a voltage applied to the second electrodeis decreased while a voltage applied to the first electrode is kept thesame.

In an embodiment, at the plurality of sub-transparent levels, as thegray scale level is increased, voltages applied to both of the firstelectrode and the second electrode are decreased.

In an embodiment, the gray scale level group includes 256 gray scalelevels; and the number of the plurality of sub-transparent levels is 20or greater.

In an embodiment, a potential difference between the first electrode andthe second electrode at each of the gray scale levels in the gray scalelevel group is 60% or less of a potential difference between the secondelectrode and the third electrode in the black display state.

In an embodiment, at a gray scale level, in the gray scale level group,at which the potential difference between the first electrode and thesecond electrode is maximum, the potential difference between the firstelectrode and the second electrode is 30% or greater of the potentialdifference between the second electrode and the third electrode in theblack display state.

In an embodiment, a voltage applied to the first electrode is decreasedas the gray scale level is increased from the lowest level to the whitelevel; and a voltage applied to the second electrode is kept the same asthe gray scale level is increased from the lowest level to a halftonelevel, and is decreased as the gray scale level is increased from thehalftone level to the white level.

In an embodiment, liquid crystal molecules in the liquid crystal layerassume twisted alignment in the transparent display state.

In an embodiment, the first electrode includes a plurality of slitsextending in a predetermined direction; and in the white display stateand the transparent display state, liquid crystal molecules at, and inthe vicinity of, a central portion of the liquid crystal layer in athickness direction are aligned to be generally perpendicular to thepredetermined direction.

In an embodiment, the liquid crystal layer contains liquid crystalmolecules having positive dielectric anisotropy.

In an embodiment, the liquid crystal display device having theabove-described structure further includes an illumination elementdirecting light of a plurality of colors including red light, greenlight and blue light in a switched manner toward the liquid crystaldisplay panel.

In an embodiment, the liquid crystal display device provides colordisplay in a field sequential system.

In an embodiment, the liquid crystal display panel does not include acolor filter.

Advantageous Effects of Invention

An embodiment of the present invention provides a liquid crystal displaydevice that has a high response characteristic and also provides a highdisplay quality and is preferably usable as a see-through displaydevice.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view schematically showing a liquid crystaldisplay device 100 in an embodiment according to the present invention.

FIG. 2 is a plan view schematically showing the liquid crystal displaydevice 100 in the embodiment according to the present invention.

FIG. 3 is a plan view showing an example of specific line structure of arear substrate 10 in the liquid crystal display device 100.

FIG. 4(a) and FIG. 4(b) are respectively a cross-sectional view and aplan view showing an alignment state of liquid crystal molecules 31 in ablack display state of the liquid crystal display device 100.

FIG. 5(a) and FIG. 5(b) are respectively a cross-sectional view and aplan view showing an alignment state of the liquid crystal molecules 31in a white display state of the liquid crystal display device 100.

FIG. 6(a) and FIG. 6(b) are respectively a cross-sectional view and aplan view showing an alignment state of the liquid crystal molecules 31in a transparent display state of the liquid crystal display device 100.

FIG. 7 is a cross-sectional view showing an alignment state of theliquid crystal molecules 31 in a halftone display state of the liquidcrystal display device 100.

FIG. 8 provides cross-sectional views schematically showing a liquidcrystal display device 800 in a comparative example; FIG. 8(a) shows ablack display state, and FIG. 8(b) shows a white display state.

FIG. 9 schematically shows that the display is blurred (visuallyrecognized double).

FIG. 10 is a graph showing a simple example of gray scale levelsettings.

FIG. 11 shows a state where noise is generated.

FIG. 12 is a graph showing an example of gray scale level settings inthe liquid crystal display device 100 in an embodiment according to thepresent invention.

FIG. 13 is a graph showing an example of voltage settings (relationshipbetween the upper voltage/the lower voltage and the gray scale level)for realizing a sub-transparent display state.

FIG. 14 is a cross-sectional view showing an alignment state of theliquid crystal molecules 31 in the sub-transparent display state.

FIG. 15 is a graph showing another example of voltage settings(relationship between the upper voltage/the lower voltage and the grayscale level) for realizing the sub-transparent display state.

FIG. 16 is a cross-sectional view showing an alignment state of theliquid crystal molecules 31 in the sub-transparent display state.

FIG. 17 is a graph showing another example of gray scale level settingsin the liquid crystal display device 100 in an embodiment according tothe present invention.

FIG. 18 is a graph showing an example of voltage settings (relationshipbetween the upper voltage/the lower voltage and the gray scale level)with which abnormal alignment change may occur at the time of gray scalelevel transition.

FIG. 19 schematically shows an alignment change from the black displaystate via the halftone display state to the white display state in thecase where the voltage settings shown in FIG. 18 are adopted.

FIG. 20 schematically shows an alignment change from the white displaystate via the halftone display state to the black display state in thecase where the voltage settings shown in FIG. 18 are adopted.

FIG. 21(a) and FIG. 21(b) are each a graph provided to describe amechanism by which the abnormal alignment change occurs.

FIG. 22 is a graph showing an example of settings of the upper voltageand the lower voltage with which the abnormal alignment change may besuppressed.

FIG. 23 is a graph showing response waveforms (time vs. brightnessrelationships) of rise response.

FIG. 24 is a graph showing waveform integrated values in a second halfof period P1 shown in FIG. 23 (132.1 to 134.2 ms).

FIG. 25 is a graph showing response waveforms (time vs. brightnessrelationships) of decay response.

FIG. 26 is a graph showing waveform integrated values in a second halfof period P2 shown in FIG. 25 (162.1 to 164.2 ms).

FIG. 27 is a graph showing another example of settings of the uppervoltage and the lower voltage with which the abnormal alignment changemay be suppressed.

FIG. 28 is a graph showing an example of settings of the upper voltageand the lower voltage with which the abnormal alignment change may besuppressed in the liquid crystal display device 100.

FIG. 29 is a cross-sectional view schematically showing another liquidcrystal display device 100′ in an embodiment according to the presentinvention.

FIG. 30 is a plan view schematically showing the another liquid crystaldisplay device 100′ in the embodiment according to the presentinvention.

FIG. 31(a) and FIG. 31(b) are respectively a cross-sectional view and aplan view showing an alignment state of liquid crystal molecules 31 inthe black display state of the liquid crystal display device 100′.

FIG. 32(a) and FIG. 32(b) are respectively a cross-sectional view and aplan view showing an alignment state of the liquid crystal molecules 31in the white display state of the liquid crystal display device 100′.

FIG. 33(a) and FIG. 33(b) are respectively a cross-sectional view and aplan view showing an alignment state of the liquid crystal molecules 31in the transparent display state of the liquid crystal display device100′.

FIG. 34(a) and FIG. 34(b) are respectively an isometric view and across-sectional view schematically showing another structure of theliquid crystal display device 100.

FIG. 35 is a graph showing the luminance in green single-color displayand the luminance in the steady state.

FIG. 36 is a graph showing the luminance in the single-color display inthe case where levels 236 to 254 are set to sub-transparent levels.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings. The present invention is not limited to anyof the following embodiments.

With reference to FIG. 1 and FIG. 2, a liquid crystal display device 100in this embodiment will be described. FIG. 1 is a cross-sectional viewschematically showing the liquid crystal display device 100, and FIG. 2is a plan view schematically showing the liquid crystal display device100.

As shown in FIG. 1, the liquid crystal display device 100 includes aliquid crystal display panel 1 and an illumination element 2. The liquidcrystal display device 100 includes a plurality of pixels arrayed in amatrix. As described below, the liquid crystal display device 100provides color display in a field sequential system.

The liquid crystal display panel 1 includes a first substrate 10 and asecond substrate 20 facing each other, and a liquid crystal layer 30provided between the first substrate 10 and the second substrate 20.Among the first substrate 10 and the second substrate 20, the firstsubstrate 10 located relatively on a rear side will be referred to as a“rear substrate”, and the second substrate 20 located relatively on afront side will be referred to as a “front substrate”.

The rear substrate 10 includes a first electrode 11 provided in each ofthe plurality of pixels, and a second electrode 12 generating a lateralelectric field in the liquid crystal layer 30 together with the firstelectrode 11. The first electrode 11 is located above the secondelectrode 12 with an insulating layer 13 being provided therebetween. Inother words, the second electrode 12 is located below the firstelectrode 11 with the insulating layer 13 being provided therebetween.In the following description, among the first electrode 11 and thesecond electrode 12, the first electrode 11 located relatively on theupper side will be referred to as an “upper electrode” and the secondelectrode 12 located relatively on the lower side will be referred to asa “lower electrode”. The lower electrode 12, the insulating layer 13 andthe upper electrode 11 are supported by a transparent substrate (e.g.,glass substrate) 10 a having an insulating property.

As shown in FIG. 1 and FIG. 2, the upper electrode 11 includes aplurality of slits 11 a extending in a predetermined direction D and aplurality of branched portions 11 b (comb teeth) extending parallel tothe direction D in which the slits 11 a extend (hereinafter, thedirection D will also be referred to as a “slit direction”). The numberof the slits 11 a and the branched portions 11 b are not limited tothose shown in FIG. 1 and FIG. 2. There is no specific limitation onwidth S of each of the slits 11 a. The width S of each slit 11 a istypically 2 μm or greater and 10 μm or less. There is no specificlimitation either on width L of each of the branched portions 11 b. Thewidth L of each branched portion 11 b is typically 2 μm or greater and10 μm or less. The upper electrode 11 is formed of a transparentconductive material (e.g., ITO).

The lower electrode 12 does not include any slit. Namely, the lowerelectrode 12 is a so-called solid electrode. The lower electrode 12 isformed of a transparent conductive material (e.g., ITO).

There is no specific limitation on the material of the insulating layer13. The insulating layer 13 may be formed of, for example, an inorganicmaterial such as silicon oxide (SiO₂), silicon nitride (SiN_(x)) or thelike or an organic material such as a photosensitive resin or the like.

The front substrate 20 includes a third electrode 21 provided to facethe upper electrode (first electrode) 11 and the lower electrode (secondelectrode) 12 (hereinafter, the third electrode will be referred to as a“counter electrode”). The counter electrode 21 is supported by atransparent substrate (e.g., glass substrate) 20 a having an insulatingproperty.

The counter electrode 21 generates a vertical electric field in theliquid crystal layer 30 together with the upper electrode 11 and thelower electrode 12. The counter electrode 21 is formed of a transparentconductive material (e.g., ITO).

Although not shown in FIG. 1, a dielectric layer (overcoat layer) maybeformed on the counter electrode 21. The overcoat layer is provided toweaken the vertical electric field unavoidably generated when thelateral electric field is generated. The overcoat layer is formed of,for example, a photosensitive resin.

The liquid crystal layer 30 contains liquid crystal molecules 31 havingpositive dielectric anisotropy. Namely, the liquid crystal layer 30 isformed of a positive liquid crystal material. In FIG. 1 and FIG. 2, theliquid crystal molecules 31 are aligned in the state where no voltage isapplied to the liquid crystal layer 30.

The liquid crystal display panel 1 further includes a pair of horizontalalignment films 14 and 24 provided to face each other with the liquidcrystal layer 30 being provided therebetween. One of the pair ofhorizontal alignment films 14 and 24, specifically, the horizontalalignment film 14 (hereinafter, may be referred to as a “firsthorizontal alignment film”), is formed on a surface of the rearsubstrate 10 on the side of the liquid crystal layer 30. The other ofthe pair of horizontal alignment films 14 and 24, specifically, thehorizontal alignment film 24 (hereinafter, may be referred to as a“second horizontal alignment film”), is formed on a surface of the frontsubstrate 20 on the side of the liquid crystal layer 30.

The first horizontal alignment film 14 and the second horizontalalignment film 24 are each alignment-processed and thus have analignment control force that aligns the liquid crystal molecules 31 inthe liquid crystal layer 30 in a predetermined direction (referred to asa “pretilt direction”). The alignment process may be, for example, arubbing process or an optical alignment process.

The pretilt directions respectively controlled by the first horizontalalignment film 14 and the second horizontal alignment film 24 are setsuch that the liquid crystal molecules 31 assume twisted alignment inthe state where no voltage is applied to the liquid crystal layer 30 (inthe state where no electric field is generated). Specifically, thepretilt directions respectively controlled by the first horizontalalignment film 14 and the second horizontal alignment film 24 have anangle of about 45 degrees with respect to the slit direction D. Thepretilt direction controlled by the second horizontal alignment film 24has an angle of 90 degrees with respect to the pretilt directioncontrolled by the first horizontal alignment film 14. Therefore, in thestate where no voltage is applied to the liquid crystal layer 30, theliquid crystal molecules 31 are twisted at 90 degrees.

The liquid crystal display panel 1 further includes a pair ofpolarization plates 15 and 25 provided to face each other with theliquid crystal layer 30 being provided therebetween. One of the pair ofpolarization plates 15 and 25, specifically, the polarization plate 15(hereinafter, also referred to a “first polarization plate”), has atransmission axis (polarization axis) 15 a, and the other of the pair ofpolarization plates 15 and 25, specifically, the polarization plate 25(hereinafter, also referred to as a “second polarization plate”), has atransmission axis (polarization axis) 25 a. As shown in FIG. 2, thetransmission axes 15 a and 25 a are generally perpendicular to eachother. Namely, the polarization plates 15 and 25 are located in acrossed-Nicols state. The transmission axis 15 a of the firstpolarization plate 15 and the transmission axis 25 a of the secondpolarization plate 25 are generally parallel or generally perpendicularto the pretilt directions respectively controlled by the firsthorizontal alignment film 14 and the second horizontal alignment film24. Therefore, the transmission axis 15 a of the first polarizationplate 15 and the transmission axis 25 a of the second polarization plate25 each have an angle of about 45 degrees with respect to the slitdirection D.

The illumination element (also referred to as a “backlight unit”) 2 islocated on the rear side of the liquid crystal display panel 1. Theillumination element 2 is capable of directing light of a plurality ofcolors including red light, green light and blue light in a switchedmanner toward the liquid crystal display panel 1.

The illumination element 2 may be, for example, of an edge light systemas shown in FIG. 1. The illumination element 2 of the edge light systemincludes a light source unit 2 a and a light guide plate 2 b. The lightsource unit 2 a may emit light of a plurality of colors including redlight, green light and blue light. The light source unit 2 a includes,for example, a red LED, a green LED and a blue LED. The light guideplate 2 b guides the color light emitted from the light source unit 2 atoward the liquid crystal display panel 1.

The liquid crystal display device 100 provides color display in thefield sequential system. Therefore, the liquid crystal display panel 1does not include any color filter.

When a predetermined voltage is applied between the upper electrode 11and the lower electrode 12 (namely, when a predetermined potentialdifference between the upper electrode 11 and the lower electrode 12 isgiven), a lateral electric field (fringe field) is generated in theliquid crystal layer 30. The“lateral electricfield” is an electric fieldincluding a component parallel to the substrate surface. The directionof the lateral electric field generated by the upper electrode 11 andthe lower electrode 12 is generally perpendicular to the slit directionD.

By contrast, when a predetermined voltage is applied between the counterelectrode 21 and the upper electrode 11/the lower electrode 12 (namely,when a predetermined potential difference between the counter electrode21 and the upper electrode 11/the lower electrode 12 is given), avertical electric field is generated. The “vertical electric field” isan electric field directed generally parallel to the normal to thesubstrate surface.

The liquid crystal display device 100 has a structure capable ofcontrolling the strength of each of the lateral electric field and thevertical electric field for each of the pixels. Typically, the liquidcrystal display device 100 has a structure capable of supplying adifferent voltage to each of the upper electrode 11 and the lowerelectrode 12 on a pixel-by-pixel basis. Specifically, the upperelectrode 11 and the lower electrode 12 are both provided for each ofthe pixels, and each pixel includes a switching element (e.g., thin filmtransistor; not shown) electrically connected with the upper electrode11 and a switching element (e.g., thin film transistor; not shown)electrically connected with the lower electrode 12. Predeterminedvoltages are respectively supplied to the upper electrode 11 and thelower electrode 12 via the corresponding switching elements. The counterelectrode 21 is formed as a single continuous conductive filmcorresponding to all the pixels. Therefore, a common potential isapplied to the counter electrode 21 in all the pixels.

FIG. 3 shows an example of specific line structure of the rear substrate10. In the structure shown in FIG. 3, each pixel includes a first TFT16A corresponding to the upper electrode 11 and a second TFT 16Bcorresponding to the lower electrode 12.

A gate electrode 16 g of each of the first TFT 16A and the second TFT16B is electrically connected with a gate bus line (scanning line) 17. Aportion of the gate bus line 17 that overlaps a channel region of eachof the first TFT 16A and the second TFT 16B acts as the gate electrodes16 g. Source electrodes 16 s of the first TFT 16A and the second TFT 16Bare electrically connected with source bus lines (signal line) 18respectively. A portion branched from each of the source bus lines 18acts as the source electrode 16 s. A drain electrode 16 d of the firstTFT 16A is electrically connected with the upper electrode 11. Bycontrast, a drain electrode 16 d of the second TFT 16B is electricallyconnected with the lower electrode 12. The line structure of the rearsubstrate 10 is not limited to that shown in FIG. 3.

In the liquid crystal display device 100 in this embodiment, each of theplurality of pixels may exhibit, in a switched manner, a “black displaystate” in which black display is provided in the state where a verticalelectric field is generated in the liquid crystal layer 30, a “whitedisplay state” in which white display is provided in the state where alateral electric field is generated in the liquid crystal layer 30, anda “transparent display state” in which the rear side of the liquidcrystal display panel 1 (i.e., background) is seen through in the statewhere no voltage is applied to the liquid crystal layer 30.

Hereinafter, with reference to FIG. 4, FIG. 5 and FIG. 6, the blackdisplay state, the white display state and the transparent display statewill be described in more detail.

FIG. 4(a) and FIG. 4(b) each show an alignment state of the liquidcrystal molecules 31 in the black display state. In the black displaystate, a predetermined voltage is applied between the counter electrode21 and the upper electrode 11/the lower electrode 12 (for example,potentials of 7 V, 7.5 V and 0 V are given to the upper electrode 11,the lower electrode 12 and the counter electrode 21 respectively), and avertical electric field is generated in the liquid crystal layer 30.FIG. 4(a) schematically shows lines of electric force in this state withdashed lines.

In the black display state, as shown in FIG. 4(a) and FIG. 4(b), theliquid crystal molecules 31 in the liquid crystal layer 30 are alignedto be generally vertical to the substrate surface (surfaces of the rearsubstrate 10 and the front substrate 20) (namely, aligned to begenerally parallel to the normal to the liquid crystal layer 30). Theliquid crystal molecules 31 in the close vicinity of the firsthorizontal alignment film 14 and the second horizontal alignment film 24are strongly influenced by the alignment control force of the firsthorizontal alignment film 14 and the second horizontal alignment film 24and thus are kept aligned to be generally parallel to the substratesurface. However, such liquid crystal molecules 31 are generallyparallel or generally perpendicular to the transmission axis 15 a of thefirst polarization plate 15, and thus do not give phase differencealmost at all to light incident on the liquid crystal layer 30 via thefirst polarization plate 15 and do not decrease the contrast ratioalmost at all.

FIG. 5(a) and FIG. 5(b) each show an alignment state of the liquidcrystal molecules 31 in the white display state. In the white displaystate, a predetermined voltage is applied between the upper electrode 11and the lower electrode 12 (for example, potentials of 0 V, 7.5 V and 0V are given to the upper electrode 11, the lower electrode 12 and thecounter electrode 21 respectively), and a lateral electric field (fringefield) is generated in the liquid crystal layer 30. FIG. 5(a)schematically shows lines of electric force in this state with dashedlines.

In the white display state, as shown in FIG. 5(a) and FIG. 5(b), theliquid crystal molecules 31 in the liquid crystal layer 30 are alignedto be generally parallel to the substrate surface (namely, aligned to begenerally vertical to the normal to the liquid crystal layer 30). Morespecifically, the liquid crystal molecules 31 in the vicinity of thefirst horizontal alignment film 14 and the liquid crystal molecules 31in the vicinity of the second horizontal alignment film 24 are alignedto have an angle of about 90 degrees with respect to each other. As aresult, the liquid crystal molecules 31 at, and in the vicinity of, acentral portion of the liquid crystal layer 30 in a thickness directionare aligned to be generally perpendicular to the direction D in whichthe slits 11 a of the upper electrode 11 extend (generally perpendicularin the slit direction D). Therefore, the average alignment direction ofthe bulk liquid crystal portion is generally perpendicular to the slitdirection D (namely, has an angle of about 45 degrees with respect tothe transmission axes 15 a and 25 a of the first polarization plate 15and the second polarization plate 25).

FIG. 6(a) and FIG. 6(b) each show an alignment state of the liquidcrystal molecules 31 in the transparent display state. In thetransparent display state, no voltage is applied to the liquid crystallayer 30 (for example, a potential of 0 V is given to all of the upperelectrode 11, the lower electrode 12 and the counter electrode 21), andneither a vertical electric field nor a lateral electric field isgenerated in the liquid crystal layer 30.

In the transparent display state, as shown in FIG. 6(a) and FIG. 6(b),the liquid crystal molecules 31 in the liquid crystal layer 30 assumetwisted alignment. Namely, the liquid crystal molecules 31 are alignedto be generally parallel to the substrate surface (namely, generallyvertical to the normal to the liquid crystal layer 30). The liquidcrystal molecules 31 in the vicinity of the first horizontal alignmentfilm 14 and the liquid crystal molecules 31 in the vicinity of thesecond horizontal alignment film 24 are aligned to have an angle ofabout 90 degrees with respect to each other. As a result, the liquidcrystal molecules 31 at, and in the vicinity of, the central portion ofthe liquid crystal layer 30 in the thickness direction are aligned to begenerally perpendicular to the slit direction D. Therefore, the averagealignment direction of the bulk liquid crystal portion is generallyperpendicular to the slit direction D (namely, has an angle of about 45degrees with respect to the transmission axes 15 a and 25 a of the firstpolarization plate 15 and the second polarization plate 25). Each of thepixels in the liquid crystal display device 100 has a highest lighttransmittance in this transparent display state (namely, higher lighttransmittance than in the black display state or the white displaystate).

Each of the plurality of pixels in the liquid crystal display device 100may exhibit a “halftone display state” in which display is provided at aluminance corresponding to a halftone as shown in FIG. 7, in addition tothe black display state, the white display state and the transparentdisplay state described above. In the halftone display state, a desiredtransmittance may be realized by adjusting the strength of the lateralelectric field (fringe field) generated in the liquid crystal layer 30(by, for example, giving a potential of 0 V to the counter electrode 21,a potential of 7.5 V to the lower electrode 12, and a potentialexceeding 0 V and less than 7 V to the upper electrode 11).

As described above, the liquid crystal display device 100 in thisembodiment provides color display in the field sequential system.Therefore, the liquid crystal display panel 1 does not need a colorfilter. This improves the light utilization factor. Also in the liquidcrystal display device 100, a vertical electric field is generated inthe liquid crystal layer 30 in the black display state and a lateralelectric field is generated in the liquid crystal layer 30 in the whitedisplay state. Therefore, a torque by voltage application acts on liquidcrystal molecules 31 in both of the fall (transition from the whitedisplay state to the black display state) and the rise (transition fromthe black display state to the white display state), and thus a highspeed response characteristic is provided.

In the liquid crystal display device 100 in this embodiment, the pixelsmay each exhibit the transparent display state in which no voltage isapplied to the liquid crystal layer 30, in addition to the black displaystate and the white display state. Displaying the background in thetransparent display prevents the problem that the background is blurred(visually recognized double). Hereinafter, reasons why this problem (thedisplay is blurred and visually recognized double) occurs in the liquidcrystal display devices in Patent Documents 1 through 3 will bedescribed by way of a liquid crystal display device in a comparativeexample.

FIG. 8(a) and FIG. 8(b) respectively show a liquid crystal displaydevice 800 in a comparative example in the black display state and thewhite display state. The liquid crystal display device 800 in thecomparative example has the same structure as that shown in FIG. 1 andFIG. 2 of Patent Document 3.

The liquid crystal display device 800 includes an array substrate 810, acounter substrate 820 and a liquid crystal layer 830 providedtherebetween. The array substrate 810 includes a glass substrate 810 a,and a lower electrode 812, an insulating layer 813 and a pair of combelectrodes (upper electrodes) 817 and 828 stacked on the glass substrate810 a in this order. Meanwhile, the counter substrate 820 includes aglass substrate 820 a and a counter electrode 821 provided on the glasssubstrate 820 a.

The liquid crystal layer 830 contains liquid crystal molecules 831having positive dielectric anisotropy. In the liquid crystal displaydevice 800, the liquid crystal molecules 831 in the liquid crystal layer830 assume a vertical alignment state in the state where no voltage isapplied.

In the liquid crystal display device 800 in the comparative example, forproving black display, a predetermined voltage is applied between thecounter electrode 821 and the lower electrode 812/upper electrodes (pairof comb electrodes) 817 and 818 (for example, a potential of 7 V isgiven to the counter electrode 821, and a potential of 14 V is given tothe lower electrode 812 and the upper electrodes 817 and 818), and avertical electric field is generated in the liquid crystal layer 830. Asa result, as shown in FIG. 8(a), the liquid crystal molecules 831 arealigned to be generally vertical to the substrate surface.

In the liquid crystal display device 800 in the comparative example, forproving white display, a predetermined voltage is applied between thepair of comb electrodes 817 and 818 (for example, a potential of 0 V isgiven to one of the comb electrodes, specifically, the comb electrode817, and a potential of 14 V is given to the other of the combelectrodes, specifically, the comb electrode 818), and a lateralelectric field is generated in the liquid crystal layer 830. As aresult, as shown in FIG. 8(b), the liquid crystal molecules 831 arealigned as being inclined with respect to the normal to the substratesurface.

In the case where the liquid crystal display device 800 in thecomparative example is simply used as a see-through display device,see-through display is provided, namely, display in which the backgroundis seen though is provided, in the white display state in which thelight transmittance of the pixels is high. However, the white displaystate is realized by applying a voltage to the liquid crystal layer 830to align the liquid crystal molecules 830. Therefore, there occurs arefractive index distribution in each pixel. As a result, light L fromthe rear side is scattered (namely, the advancing direction of the lightL is changed; see FIG. 8(b)) by the refractive index distribution), andthus the background is blurred. As a result, as shown in FIG. 9, aviewer V viewing the background BG via a see-through display device STDPvisually recognizes the background double.

As described above, when see-through display is provided in the whitedisplay state in which a voltage is applied to the liquid crystal layer,the display is blurred (visually recognized double). By contrast, theliquid crystal display device 100 in this embodiment provides backgrounddisplay (see-through display) in the state where no voltage is appliedto the liquid crystal layer 30 (in the transparent display state).Therefore, a viewer viewing the background via the liquid crystaldisplay device 100 visually recognizes the background clearly. Thus, thedisplay is prevented from being blurred (from being visually recognizeddouble), and the quality of the see-through display is improved.

In the liquid crystal display device 100 in this embodiment, a grayscale level group including gray scale levels from the lowest level tothe higher level (set of all the gray scale levels) includes a “whitelevel” corresponding to the white display state, a “transparent level”corresponding to the transparent display state and having a higherluminance than that of the white level, and a plurality of“sub-transparent levels” each having a luminance higher than that of thewhite level but lower than that of the transparent level. Since the grayscale level group is set in this manner, the decline in the displayquality caused by a difference between the white level and thetransparent level is suppressed. Hereinafter, this will be describedmore specifically.

A driving circuit for a general liquid crystal display device includesan 8-bit driver IC, and generates an output voltage for 256 levels(levels 0 to 255). In a general liquid crystal display device, level 0is assigned to the black display state, levels 1 through 254 areassigned to the halftone display state, and level 255 is assigned to thewhite display state.

Now, it is assumed that in the liquid crystal display device 100 in thisembodiment, the gray scale level group is set not to include theabove-described sub-transparent levels. In this case, for example, level0 (lowest level) is assigned to the black display state, levels 1 to 253are assigned to the halftone display state, level 254 is assigned to thewhite display state, and level 255 is assigned to the transparentdisplay state (namely, as shown in FIG. 10, levels 0 to 254 are set tovideo display levels, and level 255 is set to the transparent level). Inthis manner, the black display state, the halftone display state, thewhite display state and the transparent display state are switched toeach other. However, in this case, noise may be generated in display forthe following reason.

For, for example, storing a moving image file, it is common to compressthe file because the file has a very large capacity without beingcompressed. For displaying the moving image file, the compressed file isdecompressed before being displayed. At this point, it is almostimpossible to decompress the file with no error. Therefore, there is aslight error between the decompressed file and the original file. Thismay result in the following situation: at, or in the vicinity of, aborder between a region where the video is to be displayed (videodisplay region) and a region where transparent display is provided(transparent display region), pixels that should exhibit the transparentdisplay state but actually exhibit the white display state are present.In the liquid crystal display device 100, the luminance of the pixels inthe transparent display state and the luminance of the pixels in thewhite display state may be significantly different from each other. Forexample, the present inventors produced a panel of certainspecifications for a test. In this panel, as shown in FIG. 10, theluminance at the transparent level was higher by about 20% than theluminance at the white level. Therefore, if pixels that should exhibitthe transparent display state but actually exhibit the white displaystate are present in the transparent display region, such pixels arevisually recognized as noise clearly because these pixels provide darkerdisplay than pixels around these pixels.

FIG. 11 shows display when such noise is generated. In the example shownin FIG. 11, the circular region at the center of the display screen isthe video display region, and the remaining region is the transparentdisplay region. As shown in FIG. 11, noise is visually recognized at, orin the vicinity of, the border between the video display region and thetransparent display region.

Such a decline in the display quality is a problem unique to a liquidcrystal display device in which the pixels are capable of providing thetransparent display state in addition to the black display state and thewhite display state. In a general liquid crystal display device, thereis no large luminance difference between level 254 and level 255.Therefore, even if an error is caused by the compression anddecompression of a moving image file and as a result, pixels that shoulddisplay level 255 actually display level 254, such a difference is notconspicuous almost at all. By contrast, in a liquid crystal displaydevice in which the pixels are capable of exhibiting the transparentdisplay state, the luminance difference between the white display stateat level 254 and the transparent display state at level 255 issignificant. Therefore, if there are pixels exhibiting level 254 or lessin the transparent display state, noise is not likely to be conspicuous.

In the liquid crystal display device 100 in this embodiment, asdescribed above, the gray scale level group including the gray scalelevels from the lowest level to the highest level includes the pluralityof sub-transparent levels having a luminance higher than that of thewhite level but lower than that of the transparent level. Namely, thegray scale level group is set to include levels having an intermediateluminance between the luminance at the white level and the luminance atthe transparent level (set to include the sub-transparent levels).Therefore, the above-described noise is not visually recognized easily.

FIG. 12 shows an example of gray scale level settings in the liquidcrystal display device 100 in this embodiment. In the example shown inFIG. 12, levels 0 to 230 are set to the video display levels, level 255is set to the transparent level, and levels 231 to 254 are set to thesub-transparent levels. Namely, level 0 (lowest level) is assigned tothe black display state, levels 1 to 229 are assigned to the halftonedisplay state, level 230 is assigned to the white display state, level255 is assigned to the transparent display state, and levels 231 to 254are assigned to a display state corresponding to the sub-transparentlevels (hereinafter, this display state will be referred to as a“sub-transparent display state”).

It is now assumed that the luminance at the white level is about 0.8times the luminance at the transparent level. In this case, as shown inFIG. 12, level 230 may be set to the white level, level 255 may be setto the transparent level, and levels 231 to 254 may be set to thesub-transparent levels, so that the white level and the transparentlevel are connected with each other smoothly with a y characteristic ofγ=2.2.

In the case where the level settings are made in this manner, even ifpixels that should display the transparent level actually display alevel other than level 255 for some reason (e.g., error caused at thetime of compression or decompression of the moving image file), noise isnot visually recognized easily (the level of the noise is at leastequivalent to that in a general liquid crystal display device).

As described above, in the liquid crystal display device 100 in thisembodiment, each pixel may provide the black display state, the whitedisplay state and the transparent display state in a switched manner. Aconventional see-through display device provides see-through display ineither the black display state or the white display state regardless ofthe type thereof (liquid crystal display device, PDLC display, organicEL display, etc.) (namely, the gray scale level corresponding to theblack display state or the white display state is assigned to thesee-through display). Therefore, see-through display is not provided atan applied voltage different from both of the voltage for the blackdisplay state and the voltage for the white display state. By contrast,in the liquid crystal display device 100 in this embodiment, each pixelmay exhibit the black display state, the white display state, and alsothe transparent display state provided at a voltage different from bothof the voltage for the black display state and the voltage for the whitedisplay state. Therefore, the display is prevented from being blurred(from being visually recognized double). In addition, in the liquidcrystal display device 100 in this embodiment, the gray scale levelgroup including the gray scale levels from the lowest level to thehighest level includes a plurality of sub-transparent levels having aluminance higher than that of the white level but lower than that of thetransparent level. Therefore, the decline in the display quality causedby the luminance difference between the white level and the transparentlevel (noise being visually recognized) is suppressed.

Now, a method for realizing the sub-transparent state will be described.

In the liquid crystal display device 100 in this embodiment, thepotential difference between the upper electrode 11 and the lowerelectrode 12 at each of the plurality of sub-transparent levels (namely,in the sub-transparent display state) is smaller than the potentialdifference between the upper electrode 11 and the lower electrode 12 atthe white level. The potential difference between the lower electrode 12and the counter electrode 21 at each of the plurality of sub-transparentlevels is smaller than the potential difference between the lowerelectrode 12 and the counter electrode 21 at the white level. Thesub-transparent display state is realized by setting the potentialdifferences between the upper electrode (first electrode) 11, the lowerelectrode (second electrode) 12 and the counter electrode (thirdelectrode) 21 in this manner. Hereinafter, this will be described inmore detail by way of specific examples.

FIG. 13 shows an example of relationship between a voltage applied tothe upper electrode 11 (upper voltage)/a voltage applied to the lowerelectrode 12 (lower voltage) and the gray scale levels. FIG. 14 shows analignment state of the liquid crystal molecules 31 in thesub-transparent display state realized by the example in FIG. 13.Although not shown in FIG. 13, the voltage applied to the counterelectrode 21 (counter voltage) is 0 V at all the gray scale levels.

In the example shown in FIG. 13, the upper voltage and the lower voltagein the black display state are respectively 7 V and 7.5 V. In a regionof the video display levels (levels 0 to 230), as the gray scale levelis increased, the upper voltage is decreased while the lower voltage iskept the same. Specifically, the upper voltage is changed (decreased)from 7 V to 0 V while the lower voltage is kept at 7.5 V. By contrast,in a region of the plurality of sub-transparent levels (levels 231 to254) and the transparent level (level 255), as the gray scale level isincreased, the lower voltage is decreased while the upper voltage iskept the same. Specifically, the lower voltage is changed (decreased)from 7.5 V to 0 V while the upper voltage is kept at 0 V.

As described above, in the region of the plurality of sub-transparentlevels, as the gray scale level is increased, the potential differencebetween the upper electrode 11 and the lower electrode 12 is decreased,and the potential difference between the lower electrode 12 and thecounter electrode 21 is also decreased. Therefore, the alignment stateof the liquid crystal molecules 31 is changed from an alignment stateclose to the white display state to an alignment state close to thetransparent display state. Thus, as the gray scale level is increased,the luminance in the sub-transparent display state is changed from aluminance close to that of the white display state to a luminance closeto that of the transparent display state.

The sub-transparent display state is realized as described above. In theexample shown in FIG. 13 (having a structure in which in the region ofthe plurality of sub-transparent levels, as the gray scale level isincreased, the lower voltage is decreased while the upper voltage iskept the same), a relatively strong fringe field (lateral electricfield) is left at levels, among the plurality of sub-transparentlevels), which are close to the white level as shown in FIG. 14.Therefore, even at the sub-transparent levels, a response speed that ishigh to a certain extent is provided.

FIG. 15 shows another example of relationship between a voltage appliedto the upper electrode 11 (upper voltage)/the voltage applied to thelower electrode 12 (lower voltage) and the gray scale levels. FIG. 16shows an alignment state of the liquid crystal molecules 31 in thesub-transparent display state realized by the example in FIG. 15.Although not shown in FIG. 15, the voltage applied to the counterelectrode 21 (counter voltage) is 0 V at all the gray scale levels.

In the example shown in FIG. 15, the upper voltage and the lower voltagein the black display state are respectively 7 V and 7.5 V. In the regionof the video display levels (levels 0 to 230), as the gray scale levelis increased, the upper voltage is decreased while the lower voltage iskept the same. Specifically, the upper voltage is changed (decreased)from 7 V to 0 V while the lower voltage is kept at 7.5 V. In themeantime, in the region of the plurality of sub-transparent levels(levels 231 to 254) and the transparent level (level 255), as the grayscale level is increased, both of the upper voltage and the lowervoltage are decreased. Specifically, the upper voltage is decreased frompredetermined voltage V_(1 to) 0 V while the lower voltage is changed(decreased) from predetermined voltage V₂ to 0 V. Specifically, theupper voltage is changed (decreased) from predetermined voltage V₁ to 0V while the lower voltage is changed (decreased) from predeterminedvoltage V₂ to 0 V. The predetermined voltages V₁ and V₂ may each be setto an appropriate value exceeding 0 V and less than 7.5 V (V₁<V₂).Namely, as the gray scale level is increased, the difference between theupper voltage and the lower voltage is decreased while the relationshipthat the lower voltage is higher than the upper voltage is maintained.

As described above, in the example shown in FIG. 15 also, in the regionof the plurality of sub-transparent levels, as the gray scale level isincreased, the potential difference between the upper electrode 11 andthe lower electrode 12 is decreased, and the potential differencebetween the lower electrode 12 and the counter electrode 21 is alsodecreased. Therefore, the alignment state of the liquid crystalmolecules 31 is changed from an alignment state close to the whitedisplay state to an alignment state close to the transparent displaystate. Thus, as the gray scale level is increased, the luminance in thesub-transparent display state is changed from a luminance close to thatof the white display state to a luminance close to that of thetransparent display state.

As described above, the sub-transparent display state is realized alsoin the example shown in FIG. 15. In the example shown in FIG. 15 (havinga structure in which in the region of the plurality of sub-transparentlevels, as the gray scale level is increased, both of the upper voltageand the lower voltage are decreased), a uniform vertical electric fieldis applied to a plane parallel to the display plane. Therefore, theliquid crystal molecules 31 are aligned uniformly in the plane. Thus,the refractive index distribution is not easily caused, and diffractioncaused by the refractive index difference in the plane is not easilycaused. As a result, clearer transparent display is realized.

It should be noted that in the example shown in FIG. 15, the lateralelectric field (fringe field) applied to the liquid crystal layer 30 atthe sub-transparent levels is weaker than the lateral electric fieldapplied in the example shown in FIG. 13. This may decrease the responsespeed at the sub-transparent levels. For this reason, in the case whereresponse speed is important, it is considered to be preferable to adoptthe settings shown in FIG. 13. In the case where clearness of thetransparent display is important, it is considered to be preferable toadopt the settings shown in FIG. 15.

In the above example, levels 231 to 254 are set to the sub-transparentlevels. The number of the sub-transparent levels is not limited to theabove. The luminance difference between the white level and thetransparent level (ratio of the luminance at the white level withrespect to the luminance at the transparent level) may vary inaccordance with the specifications of the liquid crystal display device100. Thus, the number of the sub-transparent levels may be set, inaccordance with the luminance difference between the white level and thetransparent level, such that the white level and the transparent levelare connected with each other smoothly (namely, such that a desired ycharacteristic is provided). In the case of the 256-level display system(in the case where the gray scale level group includes 256 gray scalelevels), the number of the sub-transparent levels is preferably 20 orgreater from the point of view of connecting the white level and thetransparent level with each other smoothly.

However, in the case where the number of the sub-transparent levels ismade too large in order to realize a y characteristic that smooths theconnection, the number of levels assigned to the video display (numberof the video display levels) is decreased. Therefore, in the case wherethe number of the video display levels is important, the number of thesub-transparent levels may be decreased.

FIG. 17 shows an example of gray scale level settings in the case wherethe number of the video display levels is important. In the exampleshown in FIG. 17, levels 0 to 250 are set to the video display levels,level 255 is set to the transparent level, and levels 251 to 254 are setto the sub-transparent levels. Namely, level 0 (lowest level) isassigned to the black display state, levels 1 to 249 are assigned to thehalftone display state, level 250 is assigned to the white displaystate, level 255 is assigned to the transparent display state, andlevels 251 to 254 are assigned to the sub-transparent display state. Inthis case, it is difficult to provide the γ characteristic realizingγ=2.2 at the levels higher than the video display levels. However, thevoltages at the sub-transparent levels may be set such that the whitelevel and the transparent level are connected with each other assmoothly as possible, so that noise may be made difficult to be visuallyrecognized without much spoiling the gray scale representation of thevideo display. In the case of the 256-level display system, the numberof the sub-transparent levels is preferably 10 or less from the point ofview of providing a better gray scale representation of the videodisplay. The upper voltage, the lower voltage and the counter voltage atthe plurality of sub-transparent levels may be set so as to provide ahigher response speed as described above with reference to FIG. 13, ormay be set so as to provide clearer transparent display as describedabove with reference to FIG. 15.

As described above, at the sub-transparent levels, the luminance(luminance in the steady state) is higher than that of the white levelbut the response speed is lower than that of the white level. Therefore,the border between the video display levels and the sub-transparentlevels maybe specified by measuring the luminance in single-colordisplay (by measuring the single-color luminance) in, for example, thefollowing manner.

FIG. 35 shows the luminance in green single-color display and theluminance in the steady state. As seen from FIG. 35, single-colordisplay is display in which the liquid crystal layer 30 is put into alight-transmissive state only in one of a red sub frame

R (period in which red light is directed from the illumination element 2toward the liquid crystal display panel 1), a green sub frame G (periodin which green light is directed from the illumination element 2 towardthe liquid crystal display panel 1), and a blue sub frame B (period inwhich blue light is directed from the illumination element 2 toward theliquid crystal display panel 1).

FIG. 36 shows the luminance in the single-color display in the casewhere levels 236 to 254 are set to the sub-transparent levels. At thewhite level, a lateral electric field (fringe field) strong enough torealize a sufficiently high response speed is applied to the liquidcrystal layer 30. By contrast, at the sub-transparent levels, a lateralelectric field (fringe field) weaker than that at the white level isapplied to the liquid crystal layer 30. Therefore, as seen from FIG. 36,the single-color luminance is highest at the white level. For thisreason, the border between the video display levels and thesub-transparent levels may be specified by measuring the single-colorluminance.

In the above example, the 256-level display system is described. Theembodiment of the present invention is not limited to such a system. Thepresent invention is also applicable to a system other than the256-level display system, as long as the gray scale level groupincluding the gray scale levels from the lowest level to the highestlevel includes a plurality of sub-transparent levels between the whitelevel and the transparent level.

In the case where the ratio of the potential difference between theupper electrode 11 and the lower electrode 12 at each gray scale levelfrom the lowest level to the highest level, with respect to thepotential difference between the lower electrode 12 and the counterelectrode 21 in the black display state, is a predetermined value orless (specifically, 60% or less), the occurrence of abnormal alignmentchange is suppressed as described below. Hereinafter, this will bedescribed more specifically. In the following, a case where nosub-transparent level is provided will be described first, for thesimplicity of explanation.

As a result of active studies, the present inventors have confirmed thatwhen a voltage applied to the upper electrode 11 (upper voltage) and avoltage applied to the lower electrode 12 (lower voltage) are simply setwith no specific consideration, abnormal alignment change may occur atthe time of gray scale level transition. FIG. 18 shows an example ofvoltage settings (relationship between the upper voltage/the lowervoltage and the gray scale level) with which the abnormal alignmentchange may occur. Although not shown in FIG. 18, the voltage applied tothe counter electrode 21 (counter voltage) is 0 V at all the gray scalelevels.

In the example shown in FIG. 18, as the gray scale level is increasedfrom level 0 (corresponding to the black display state) to level 254(corresponding to the white display state), the upper voltage isdecreased while the lower voltage is kept the same. Specifically, theupper voltage is changed (decreased) from 7 V to 0 V while the lowervoltage is kept at 7.5 V. At level 255 (corresponding to the transparentdisplay state), the upper voltage and also the lower voltage become 0 V.In this example, there are gray scale levels at which the potentialdifference between the upper electrode 11 and the lower electrode 12exceeds 60% of the potential difference between the lower electrode 12and the counter electrode 21 in the black display state. In the casewhere the voltage settings shown in FIG. 18 are adopted, abnormalalignment change may occur at the time of gray scale level transition.

FIG. 19 schematically shows an alignment change from the black displaystate via the halftone display state to the white display state. As seenfrom FIG. 19, as the gray scale level is increased, liquid crystalmolecules 31 twisted in a direction opposite to the proper twistingdirection and liquid crystal molecules 31 tilted in a direction oppositeto the proper tilting direction appear in addition to liquid crystalmolecules 31 aligned normally.

FIG. 20 schematically shows an alignment change from the white displaystate via the halftone display state to the black display state. As seenfrom FIG. 20, as the gray scale level is decreased, the number of theliquid crystal molecules 31 twisted in the opposite direction and theliquid crystal molecules 31 tilted in the opposite direction isdecreased.

As seen from above, in the case where the voltage settings shown in FIG.18 are adopted, abnormal alignment change may occur at the time of grayscale level transition. FIG. 19 and FIG. 20 show how white lines andblack lines caused by the abnormal alignment change extend. The abnormalalignment change occurs at a visually recognizable speed (severalhundred milliseconds to several seconds). The degree of abnormalalignment change varies inside each pixel and/or on a pixel-by-pixelbasis. Therefore, the abnormal alignment change is observed as displaynon-uniformity or roughness, which declines the display quality.

With reference to FIG. 21(a) and FIG. 21(b), an observation of thepresent inventors on a mechanism by which the abnormal alignment changeoccurs will be described. The abnormal alignment change is considered tooccur because the alignment control force provided by the lateralelectric field (fringe field) generated by the potential differencebetween the upper electrode 11 and the lower electrode 12, and thealignment control force provided by the horizontal alignment films 14and 24, do not match each other partially. As shown in FIG. 21(a), whilethe gray scale level is increased from the black display state to thewhite display state, the potential difference between the upperelectrode 11 and the lower electrode 12 exceeds a certain threshold.This is considered to cause the abnormal alignment change (herein, thisis referred to “set”). As shown in FIG. 21(b), the abnormal alignmentchange is considered to be solved (herein, this is referred to “reset”)while the gray scale level is decreased from the white display state tothe black display state.

FIG. 22 shows an example of settings of the upper voltage and the lowervoltage with which the abnormal alignment change may be suppressed. Inthis example, as shown in FIG. 22, the potential difference between theupper electrode 11 and the lower electrode 12 is set so as not to exceeda certain threshold. Namely, the ratio of the potential differencebetween the upper electrode 11 and the lower electrode 12 at each grayscale level from the lowest level to the highest level, with respect tothe potential difference between the lower electrode 12 and the counterelectrode 21 in the black display state, is set to be a predeterminedvalue or less (specifically, 60% or less).

In the example shown in FIG. 22, as the gray scale level is increasedfrom the lowest level to the highest level, the voltage applied to theupper electrode 11 is decreased. Specifically, as the gray scale levelis increased, the upper voltage is decreased from V₁ (e.g., 7 V) to 0 V.By contrast, the voltage applied to the lower electrode 12 is kept thesame as the gray scale level is increased from the lowest level to acertain halftone level, and is decreased as the gray scale level isincreased from the halftone level to the level corresponding to thewhite display state. Specifically, the lower voltage is kept at V₂(e.g., 7.5 V) as the gray scale level is increased from the lowest levelto a certain halftone level (level at which the upper voltage becomesV₃), and then is decreased from V₂ to V₄ at the same ratio as that ofthe upper voltage as the gray scale level is increased from the halftonelevel to the level corresponding to the white display state. Namely, inthis example, the potential difference between the upper electrode 11and the lower electrode 12 is set to be V₄ or less, and V₄ is 60% orless of the potential difference V₂ between the lower electrode 12 andthe counter electrode 21 in the black display state.

The present inventors made an investigation on whether the abnormalalignment change would be suppressed or not in a plurality of settingsdifferent from each other in the maximum potential difference betweenthe upper electrode 11 and the lower electrode 12. The results will bedescribed. Table 1 shows, for each of settings 1 through 5, the uppervoltage and the lower voltage in the black display state, the uppervoltage when the lower voltage starts to be decreased (V₃ in FIG. 22),the upper voltage and the lower voltage in the white display state (V₄in FIG. 22), and whether the abnormal alignment change is suppressed ornot. In Table 1, “×” indicates that the abnormal alignment changeoccurred, and “◯” indicates that the abnormal alignment change wassuppressed. “Δ” indicates that the abnormal alignment change wasbasically suppressed, but the abnormal alignment change occurred whenthe gray scale level was changed to the level corresponding to the whitedisplay state in a certain manner. The investigation results shown inTable 1 were obtained under the following conditions: width S of theslit 11 a of the upper electrode 11: 3 μm; width L of the branchedportion 11 b: 4 μm; and dielectric anisotropy 46 of the liquid crystalmaterial used for the liquid crystal layer 30: 17.8.

TABLE 1 BLACK DISPLAY WHITE DISPLAY STATE UPPER VOLTAGE STATE WHETHERUPPER LOWER WHEN LOWER VOLTAGE UPPER LOWER ABNORMAL VOLTAGE VOLTAGESTARTS TO BE DECREASED VOLTAGE VOLTAGE ALIGNMENT (V) (V) (V) (V) (V) ISSUPPRESSED SETTING 1 7 7.5 — 0 7.5 X SETTING 2 2 5.5 X SETTING 3 3 4.5 XSETTING 4 3.5 4 Δ SETTING 5 4 3.5 ◯

As seen from Table 1, in settings 1, 2 and 3, in which the maximumpotential difference between the upper electrode 11 and the lowerelectrode 12 (same potential as the lower voltage in the white displaystate) was 7.5 V, 5.5 V and 4.5 V, the abnormal alignment changeoccurred. By contrast, in setting 4, in which the maximum potentialdifference between the upper electrode 11 and the lower electrode 12 was4 V, the abnormal alignment change was suppressed. In setting 5, inwhich the maximum potential difference between the upper electrode 11and the lower electrode 12 was 3.5 V, the abnormal alignment change wasfurther suppressed.

Regarding each of settings 1 through 5, an investigation was made onwhether the abnormal alignment change would be suppressed or not whilethe manner of transition of the gray scale level to the levelcorresponding to the white display state in various manners was changed.Table 2 shows the investigation results obtained when the transparentdisplay state was suddenly changed to the white display state, when theblack display state was suddenly changed to the white display state,when the black display state was gradually changed to the white displaystate, and when the transparent display state was gradually changed tothe white display state. In Table 2, “×” indicates that the abnormalalignment change occurred, and “◯” indicates that the abnormal alignmentchange was suppressed.

TABLE 2 LOWER VOLTAGE IN WHITE DISPLAY STATE MANNER OF (=MAXIMUMPOTENTIAL DIFFERENCE BETWEEN TRANSITION TO UPPER ELECTRODE AND LOWERELECTRODE) WHITE DISPLAY STATE SETTING 1: 7.5 V SETTING 2: 5.5 V SETTING3: 4.5 V SETTING 4: 4 V SETTING 5: 3.5 V SUDDENLY FROM X X X ◯ ◯TRANSPARENT DISPLAY STATE SUDDENLY FROM X X X ◯ ◯ BLACK DISPLAY STATEGRADUALLY FROM X X X X ◯ BLACK DISPLAY STATE GRADUALLY FROM X X X X ◯TRANSPARENT DISPLAY STATE

As seen from Table 2, in settings 1 through 3, the abnormal alignmentchange occurred in any of the manners of transition. In setting 4, theabnormal alignment change was suppressed when the transparent displaystate was suddenly changed to the white display state and when the blackdisplay state was suddenly changed to the white display state, but theabnormal alignment change occurred when the black display state wasgradually changed to the white display state and when the transparentdisplay state was gradually changed to the white display state. Bycontrast, in setting 5, the abnormal alignment change was suppressed inany of the manners of transition.

An investigation was made on whether the abnormal alignment change wouldbe suppressed or not while the specification of the liquid crystaldisplay panel 1 was changed. Table 3 shows the investigation results.Table 3 shows, in each of specifications 1 through 4, whether theabnormal alignment change was suppressed or not at various values of thedielectric anisotropy Δε of the liquid crystal material, the length L(μm) of the branched portion 11 b of the upper electrode 11 and thewidth S (μm) of the slit 11 a of the upper electrode 11, and when thelower voltage in the white display state was 3 V, 3.5 V, 4 V and 4.5 V.Specification 1 is used for the results shown in Table 1. In Table 3,“×”, “◯” and “Δ” indicate the same as those in Table 1.

TABLE 3 LOWER VOLTAGE IN WHITE DISPLAY STATE (=MAXIMUM POTENTIALDIFFERENCE Δε OF BETWEEN UPPER LIQUID ELECTRODE AND CRYSTAL L/S OF UPPERLOWER ELECTRODE) MATERIAL ELECTRODE 3 V 3.5 V 4 V 4.5 V SPECIFI- 17.84/3 — ◯ Δ X CATION 1 SPECIFI- 5/3 — ◯ ◯ Δ CATION 2 SPECIFI- 3/5 ◯ Δ X —CATION 3 SPECIFI- 20 5/3 — ◯ Δ — CATION 4

As shown in Table 3, when the maximum potential difference between theupper electrode 11 and the lower electrode 12 was 4.5 V, the abnormalalignment change was suppressed in specification 2. When the maximumpotential difference was 4 V, the abnormal alignment change wassuppressed in specifications 1, 2 and 4. When the maximum potentialdifference was 3.5 V, the abnormal alignment change was suppressed inany of specifications 1 through 4.

It is seen from these results that in the case where the potentialdifference between the upper electrode 11 and the lower electrode 12 ateach gray scale level from the lowest level to the highest level is 60%or less of the potential difference between the electrode 11 and thecounter electrode 12 in the black display state, the abnormal alignmentchange is suppressed. It is also seen that from the point of view ofsuppressing the abnormal alignment change, the potential differencebetween the upper electrode 11 and the lower electrode 12 at each grayscale level from the lowest level to the highest level is preferably 54%or less, and more preferably 47% or less, of the potential differencebetween the electrode 11 and the counter electrode 12 in the blackdisplay state.

When the maximum potential difference between the upper electrode 11 andthe lower electrode 12 is too small, the response speed may beundesirably decreased. Therefore, from the point of view of the responsespeed, it is considered to be preferable that the maximum potentialdifference between the upper electrode 11 and the lower electrode 12 isas large as possible in the range in which the abnormal alignment changeis suppressed. Specifically, at a gray scale level, among all the grayscale levels from the lowest level to the highest level, at which thepotential difference between the upper electrode 11 and the lowerelectrode 12 is maximum, the potential difference between the upperelectrode 11 and the lower electrode 12 is preferably 30% or greater,and more preferably 40% or greater, of the potential difference betweenthe lower electrode 12 and the counter electrode 21 in the black displaystate.

FIG. 23 shows the response waveforms (time vs. brightness relationships)of rise response in settings 4 and 5 shown in Table 1. FIG. 23 shows theresponse waveforms when the black display state is switched to the whitedisplay state at the maximum potential difference of 4 V (setting 4) and3.5 V (setting 5) (respectively labeled as “0-254: 4 V” and “0-254: 3.5V”). For a comparison, FIG. 23 also shows the response waveforms whenthe black display state is switched to the white display state (labeledas “0-254”) and when the black display state is switched to thetransparent display state (labeled as “0-255”) at the maximum potentialdifference of 7.5 V (setting 1). The brightness values along thevertical axis in FIG. 23 are relative values with respect to thebrightness in the transparent display state being 100 (this is alsoapplied to FIG. 25 described below).

As seen from FIG. 23, the rise response when the maximum potentialdifference is 3.5 V or 4 V (0-254: 3.5 V, 0-254: 4 V) is slightly lowerthan the rise response when the maximum potential difference is 7.5 V(0-254), but is higher than the response to the switch from the blackdisplay state to the transparent display state (0-255). Even in the casewhere the maximum potential difference between the upper electrode 11and the lower electrode 12 is made smaller to a certain degree, asufficiently high response speed is guaranteed.

FIG. 24 shows the waveform integrated values in a second half of periodP1 shown in FIG. 23 (132.1 to 134.2 ms). Herein, it is assumed toperform field sequential driving with a frame frequency of 240 Hz (1frame: about 4.2 msec.) and a duty ratio (period in which the backlightis lit on) of 50%.

As seen from FIG. 24, when the maximum potential difference is 3.5 V or4 V (0-254: 3.5 V, 0-254: 4 V), the brightness is lower by about 10%than the brightness when the maximum potential difference is 7.5 V(0-254).

FIG. 25 shows the response waveforms (time vs. brightness relationships)of decay response in settings 4 and 5 shown in Table 1. FIG. 25 showsthe response waveforms when the white display state is switched to theblack display state at the maximum potential difference of 4 V (setting4) and 3.5 V (setting 5) (respectively labeled as “0-254: 4 V” and“0-254: 3.5 V”). For a comparison, FIG. 25 also shows the responsewaveforms when the white display state is switched to the black displaystate (labeled as “0-254”) and when the transparent display state isswitched to the black display state (labeled as “0-255”) at the maximumpotential difference of 7.5 V (setting 1).

As seen from FIG. 25, the decay response when the maximum potentialdifference is 3.5 V or 4 V (0-254: 3.5 V, 0-254: 4 V) is higher than thedecay response when the maximum potential difference is 7.5 V (0-254).As can be seen, the decay response is improved by decreasing the maximumpotential difference between the upper electrode 11 and the lowerelectrode 12. It is seen from the waveforms at 158 to 160 ms that thecolor of white is made brighter by decreasing the lower voltage in thewhite display state.

FIG. 26 shows the waveform integrated values in a second half of periodP2 shown in FIG. 25 (162.1 to 164.2 ms). Herein, it is assumed toperform field sequential driving with a frame frequency of 240 Hz and aduty ratio of 50%.

As seen from FIG. 26, when the maximum potential difference is 3.5 V or4 V (0-254: 3.5 V, 0-254: 4 V), the black display is darker than whenthe maximum potential difference is 7.5 V (0-254). As can be seen, whenthe field sequential driving is performed, the occurrence of colormixing is suppressed by decreasing the maximum potential differencebetween the upper electrode 11 and the lower electrode 12.

As described above, even when the maximum potential difference betweenthe upper electrode 11 and the lower electrode 12 is decreased (evenwhen the lower voltage in the white display state is decreased), asufficiently high rise response speed is realized. In addition, thedecay response is improved, and brighter white display is realized.

FIG. 27 shows another example of settings of the upper voltage and thelower voltage.

In the example shown in FIG. 27, the voltage applied to the upperelectrode 11 is decreased as the gray scale level is increased from thelowest level to the level corresponding to the white display state.Specifically, the upper voltage is decreased from V₁ (e.g., 7 V) to 0 Vas the gray scale level is increased. The voltage applied to the lowerelectrode 12 is also decreased as the gray scale level is increased fromthe lowest level to the level corresponding to the white display state.Specifically, the lower voltage is decreased from V₂ (e.g., 7.5 V) to V₃(predetermined voltage exceeding 0 V) at a ratio lower than that of theupper voltage, as the gray scale level is increased. Namely, in thisexample, the potential difference between the upper electrode 11 and thelower electrode 12 is set to be V₃ or less, and V₃ is 60% or less of thepotential difference V₂ between the lower electrode 12 and the counterelectrode 21 in the black display state.

From the point of view of realizing high speed response, it ispreferable that as strong a lateral electric field (fringe field) aspossible is applied to the liquid crystal layer 30 at many gray scalelevels. Therefore, from the point of view of providing a high responsecharacteristic, the example shown in FIG. 22 is considered to bepreferable to the example shown in FIG. 27.

As described above, the potential difference between the upper electrode11 and the lower electrode 12 at each gray scale level is 60% or less ofthe potential difference between the lower electrode 12 and the counterelectrode 21 in the black display state. This suppresses the occurrenceof the abnormal alignment change at the time of gray scale leveltransition. In the above example described with reference to FIG. 22through FIG. 27, the sub-transparent levels are not provided.

FIG. 28 shows an example of voltage settings in the case where the grayscale level group includes a plurality of sub-transparent levels and thepotential difference between the upper electrode 11 and the lowerelectrode 12 at each gray scale level is 60% or less of the potentialdifference between the lower electrode 12 and the counter electrode 21in the black display state.

In the example shown in FIG. 28, the voltage applied to the upperelectrode 11 is decreased as the gray scale level is increased from thelowest level to the level corresponding to the white display state.Specifically, the upper voltage is decreased from V₁ (e.g. 7 V) to 0 Vas the gray scale level is increased. By contrast, the voltage appliedto the lower electrode 12 is kept the same as the gray scale level isincreased from the lowest level to a halftone level, and is decreased asthe gray scale level is increased from the halftone level to the levelcorresponding to the white display state. Specifically, the lowervoltage is kept at V₂ (e.g., 7.5 V) as the gray scale level is increasedfrom the lowest level to a halftone level (level at which the uppervoltage becomes V₃), and is decreased from V₂ to V₄ at a ratio higherthan that of the upper voltage, as the gray scale level is increasedfrom the halftone level to the level corresponding to the white displaystate. Namely, in this example, the potential difference between theupper electrode 11 and the lower electrode 12 is set to be (V₂−V₃) orless. (V₂−V₃) is 60% or less of the potential difference V₂ between thelower electrode 12 and the counter electrode 21 in the black displaystate.

FIG. 28 shows a case where the ratio of decrease in the lower voltagefrom a halftone level to the level corresponding to the white displaystate is higher than the ratio of decrease in the upper voltage.Alternatively, the ratio of decrease in the lower voltage may be equalto the ratio of decrease in the upper voltage, or may be lower than theratio of decrease in the upper voltage. Like in the example shown inFIG. 27, there may not be a period in which the lower voltage is keptthe same. The maximum potential difference between the upper electrode11 and the lower electrode 12 is 60% or less of the potential differencebetween the lower electrode 12 and the counter electrode 21 in the blackdisplay state, so that the occurrence of the abnormal alignment changeat the time of gray scale level transition is suppressed.

In the above description, in the transparent display state, the liquidcrystal molecules 31 in the liquid crystal layer 30 assume twistedalignment. Such a structure realizes clearer transparent display for thefollowing reason. When assuming twisted alignment, the liquid crystalmolecules 31 are oriented in the same direction in a plane parallel tothe display surface. Therefore, there is no diffraction caused by therefractive index difference in the plane or diffraction by the dark linecaused by the liquid crystal mode (dark line caused by a structural bodycontrolling the alignment direction or dark line by discontinuity in thealignment direction caused in the plane).

In this example, in the white display state and the transparent displaystate, the liquid crystal molecules 31 at, and in the vicinity of, thecentral portion of the liquid crystal layer 30 in the thicknessdirection are aligned to be generally perpendicular to the slitdirection D (namely, the average alignment direction of the bulk liquidcrystal portion is generally perpendicular to the slit direction D).Alternatively, the liquid crystal molecules 31 at, and in the vicinityof, the central portion of the liquid crystal layer 30 in the thicknessdirection may be aligned to be generally parallel to the slit directionD (namely, the average alignment direction of the bulk liquid crystalportion is generally parallel to the slit direction D). It should benoted that the former structure (perpendicular type structure) ispreferable to the latter structure (parallel type structure) from thepoint of view of display brightness.

Still alternatively, as in a liquid crystal display device 100′ shown inFIG. 29 and FIG. 30, the liquid crystal molecules 31 in the liquidcrystal layer 30 may assume homogeneous alignment in the transparentdisplay state.

In the liquid crystal display device 100′, the pretilt directionsrespectively controlled by the first horizontal alignment film 14 andthe second horizontal alignment state 24 are set such that the liquidcrystal molecules 31 assume homogeneous alignment in the state where novoltage is applied to the liquid crystal layer 30 (in the state where noelectric field is generated). Specifically, the pretilt directionsrespectively controlled by the first horizontal alignment film 14 andthe second horizontal alignment state 24 are generally perpendicular tothe direction in which the slits 11 a of the upper electrode 11 extend(generally perpendicular to the slit direction D). Namely, the pretiltdirection controlled by the first horizontal alignment film 14 and thepretilt direction controlled by the second horizontal alignment film 24are parallel or antiparallel to each other.

The transmission axes 15 a and 25 a of the first polarization plate 15and the second polarization plate 25 have an angle of about 45 degreeswith respect to the pretilt directions respectively controlled by thefirst horizontal alignment film 14 and the second horizontal alignmentfilm 24. Therefore, the transmission axes 15 a and 25 a of the firstpolarization plate 15 and the second polarization plate 25 have an angleof about 45 degrees with respect to the slit direction D.

FIG. 31(a) and FIG. 31(b) show an alignment state of the liquid crystalmolecules 31 in the black display state. In the black display state, apredetermined voltage is applied between the counter electrode 21 andthe upper electrode 11/the lower electrode 12 (for example, potentialsof 7 V, 7.5 V and 0 V are respectively given to the upper electrode 11,the lower electrode 12 and the counter electrode 21), and a verticalelectric field is generated in the liquid crystal layer 30. FIG. 31(a)schematically shows line of electric force in this state with dashedlines.

In the black display state, as shown in FIG. 31(a) and FIG. 31(b), theliquid crystal molecules 31 in the liquid crystal layer 30 are alignedto be generally vertical to the substrate surface (surfaces of the rearsubstrate 10 and the front substrate 20) (namely, aligned to begenerally parallel to the normal to the liquid crystal layer 30).

FIG. 32(a) and FIG. 32(b) show an alignment state of the liquid crystalmolecules 31 in the white display state. In the white display state, apredetermined voltage is applied between the upper electrode 11 and thelower electrode 12 (for example, potentials of 0 V, 7.5 V and 0 V arerespectively given to the upper electrode 11, the lower electrode 12 andthe counter electrode 21), and a lateral electric field (fringe field)is generated in the liquid crystal layer 30. FIG. 32(a) schematicallyshows line of electric force in this state with dashed lines.

In the white display state, as shown in FIG. 32(a) and FIG. 32(b), theliquid crystal molecules 31 in the liquid crystal layer 30 are alignedto be generally parallel to the substrate surface (namely, aligned to begenerally vertical to the normal to the liquid crystal layer 30). Morespecifically, the liquid crystal molecules 31 are aligned to begenerally perpendicular to the direction D in which the slits 11 a ofthe upper electrode 11 extend. Namely, the liquid crystal molecules 31are aligned to have an angle of about 45 degrees with respect to thetransmission axes 15 a and 25 a of the first polarization plate 15 andthe second polarization plate 25.

FIG. 33(a) and FIG. 33(b) show an alignment state of the liquid crystalmolecules 31 in the transparent display state. In the transparentdisplay state, no voltage is applied to the liquid crystal layer 30 (forexample, a potential of 0 V is given to all of the upper electrode 11,the lower electrode 12 and the counter electrode 21), and neither avertical electric field nor a lateral electric field is generated in theliquid crystal layer 30.

In the transparent display state, as shown in FIG. 33(a) and FIG. 33(b),the liquid crystal molecules 31 in the liquid crystal layer 30 assumehomogeneous alignment. Namely, the liquid crystal molecules 31 arealigned to be generally parallel to the substrate surface (namely,aligned to be generally vertical to the normal to the liquid crystallayer 30). More specifically, the liquid crystal molecules 31 arealigned to be generally perpendicular to the direction D in which theslits 11 a of the upper electrode 11 extend. Namely, the liquid crystalmolecules 31 are aligned to have an angle of about 45 degrees withrespect to the transmission axes 15 a and 25 a of the first polarizationplate 15 and the second polarization plate 25. In this transparentdisplay state, pixels in the liquid crystal display device 100′ have ahighest light transmittance (namely, higher light transmittance than inthe black display state or the white display state).

Also in the liquid crystal display device 100′, a vertical electricfield is generated in the liquid crystal layer 30 in the black displaystate and a lateral electric field is generated in the liquid crystallayer 30 in the white display state. Therefore, a torque by voltageapplication acts on the liquid crystal molecules 31 in both of the fall(transition from the white display state to the black display state) andthe rise (transition from the black display state to the white displaystate), and thus a high speed response characteristic is provided. Eachof the pixels may exhibit the black display state, the white displaystate, and also the transparent display state in which no voltage isapplied to the liquid crystal layer 30. Therefore, the problem that thebackground is blurred (visually recognized double) is prevented. Inaddition, since the gray scale level group including the gray scalelevels from the lowest level to the highest level (set of all the grayscale levels) includes the sub-transparent levels in addition to thewhite level and the transparent level, the decline in the displayquality caused by the luminance difference between the white level andthe transparent level is suppressed.

FIG. 1 and FIG. 29 show a structure in which the backlight unit of theedge light system as the illumination element 2 is located on the rearside of the liquid crystal display panel 1 so as to overlap the liquidcrystal display panel 1. The illumination element 2 is not limited tobeing provided in this manner.

For example, the structure shown in FIG. 34 may be adopted. In thestructure shown in FIG. 34, the liquid crystal display panel 1 and theillumination element 2 of the liquid crystal display device 100 (or theliquid crystal display device 100′) are attached to a box-shapedtransparent case 50. The case 50 having the liquid crystal display panel1 and the illumination element 2 attached thereto is used as, forexample, a showcase.

The liquid crystal display panel 1 is attached to a side surface 50 samong a plurality of side surfaces of the case 50. The illuminationelement 2 is attached to a top surface 50 t of the case 50. As describedabove, the illumination element 2 may direct light of a plurality ofcolors including red light, green light and blue light in a switchedmanner toward the liquid crystal display panel 1. From the point of viewof increasing the light utilization factor (from the point of view ofcausing light from the illumination element 2 in as much amount aspossible to be incident on the liquid crystal display panel 1), it ispreferable that an inner surface of the case 50 is light-diffusive.

In the above, color display provided in the field sequential system isdescribed. The liquid crystal display device in an embodiment accordingto the present invention is not limited to a liquid crystal displaydevice providing color display in the field sequential system. Even aliquid crystal display device including a liquid crystal display panelthat includes a color filter prevents display from being blurred (frombeing visually recognized double) as long as the pixels exhibit theblack display state, the white display state and the transparent displaystate in a switched manner.

INDUSTRIAL APPLICABILITY

An embodiment according to the present invention provides a liquidcrystal display device that has a high response characteristic and alsoprovides a high display quality and is preferably usable as asee-through display device. The liquid crystal display device(see-through display device) in an embodiment according to the presentinvention is usable as a display device for, for example, informationdisplay or digital signage.

REFERENCE SIGNS LIST

1 Liquid crystal display panel

2 Illumination element

2 a Light source unit

2 b Light guide plate

10 First substrate (rear substrate)

10 a Transparent substrate

11 First electrode (upper electrode)

11 a Slit

11 b Branched portion

12 Second electrode (lower electrode)

13 Insulating layer

14 First horizontal alignment film

15 First polarization plate

15 a Transmission axis of the first polarization plate

16A First TFT

16B Second TFT

17 Gate bus line

18 Source bus line

20 Second substrate (front substrate)

20 a Transparent substrate

21 Third electrode (counter electrode)

24 Second horizontal alignment film

25 Second polarization plate

25 a Transmission axis of the second polarization plate

30 Liquid crystal layer

31 Liquid crystal molecule

50 Case

100, 100′ Liquid crystal display device

The invention claimed is:
 1. A liquid crystal display device,comprising: a liquid crystal display panel including a first substrateand a second substrate facing each other, and a liquid crystal layerprovided between the first substrate and the second substrate; theliquid crystal display device including a plurality of pixels arrayed ina matrix; wherein: the first substrate includes a first electrodeprovided in each of the plurality of pixels and a second electrodegenerating a lateral electric field in the liquid crystal layer togetherwith the first electrode; the second substrate includes a thirdelectrode provided to face the first electrode and the second electrode,the third electrode generating a vertical electric field in the liquidcrystal layer together with the first electrode and the secondelectrode; the plurality of pixels each exhibit, in a switched manner, ablack display state where black display is provided in a state where thevertical electric field is generated in the liquid crystal layer, awhite display state where white display is provided in a state where thelateral electric field is generated in the liquid crystal layer, and atransparent display state where a rear side of the liquid crystaldisplay panel is seen through in a state where no voltage is applied tothe liquid crystal layer; and a gray scale level group including grayscale levels from a lowest level to a highest level includes: a whitelevel corresponding to the white display state, a transparent levelcorresponding to the transparent display state and having a luminancehigher than that of the white level, and a plurality of sub-transparentlevels each having a luminance higher than that of the white level andlower than that of the transparent level.
 2. The liquid crystal displaydevice according to claim 1, wherein the second electrode is providedbelow the first electrode with an insulating layer being providedbetween the first electrode and the second electrode.
 3. The liquidcrystal display device according to claim 2, wherein: a potentialdifference between the first electrode and the second electrode at eachof the plurality of sub-transparent levels is smaller than a potentialdifference between the first electrode and the second electrode at thewhite level; and a potential difference between the second electrode andthe third electrode at each of the plurality of sub-transparent levelsis smaller than a potential difference between the second electrode andthe third electrode at the white level.
 4. The liquid crystal displaydevice according to claim 3, wherein at the plurality of sub-transparentlevels, as the gray scale level is increased, a voltage applied to thesecond electrode is decreased while a voltage applied to the firstelectrode is kept the same.
 5. The liquid crystal display deviceaccording to claim 3, wherein at the plurality of sub-transparentlevels, as the gray scale level is increased, voltages applied to bothof the first electrode and the second electrode are decreased.
 6. Theliquid crystal display device according to claim 1, wherein: the grayscale level group includes 256 gray scale levels; and the number of theplurality of sub-transparent levels is 20 or greater.
 7. The liquidcrystal display device according to claim 2, wherein a potentialdifference between the first electrode and the second electrode at eachof the gray scale levels in the gray scale level group is 60% or less ofa potential difference between the second electrode and the thirdelectrode in the black display state.
 8. The liquid crystal displaydevice according to claim 7, wherein at a gray scale level, in the grayscale level group, at which the potential difference between the firstelectrode and the second electrode is maximum, the potential differencebetween the first electrode and the second electrode is 30% or greaterof the potential difference between the second electrode and the thirdelectrode in the black display state.
 9. The liquid crystal displaydevice according to claim 7, wherein: a voltage applied to the firstelectrode is decreased as the gray scale level is increased from thelowest level to the white level; and a voltage applied to the secondelectrode is kept the same as the gray scale level is increased from thelowest level to a halftone level, and is decreased as the gray scalelevel is increased from the halftone level to the white level.
 10. Theliquid crystal display device according to claim 1, wherein liquidcrystal molecules in the liquid crystal layer assume twisted alignmentin the transparent display state.
 11. The liquid crystal display deviceaccording to claim 10, wherein: the first electrode includes a pluralityof slits extending in a predetermined direction; and in the whitedisplay state and the transparent display state, liquid crystalmolecules at, and in the vicinity of, a central portion of the liquidcrystal layer in a thickness direction are aligned to be generallyperpendicular to the predetermined direction.
 12. The liquid crystaldisplay device according to claim 1, wherein the liquid crystal layercontains liquid crystal molecules having positive dielectric anisotropy.13. The liquid crystal display device according to claim 1, furthercomprising an illumination element directing light of a plurality ofcolors including red light, green light and blue light in a switchedmanner toward the liquid crystal display panel.
 14. The liquid crystaldisplay device according to claim 1, wherein the liquid crystal displaydevice provides color display in a field sequential system.
 15. Theliquid crystal display device according to claim 1, wherein the liquidcrystal display panel does not include a color filter.